1. Field of the Invention
The present invention relates to a hydraulic control apparatus of an automatic transmission, and in particular it relates to a hydraulic control apparatus of an automatic transmission capable of improving the reliability during garage shifting.
2. Description of the Related Art
A conventional hydraulic control apparatus of an automatic transmission includes a plurality of control valves for regulating hydraulic pressure from an oil pump to hydraulic control pressure corresponding to the energization amount of a linear solenoid, a plurality of shift valves for switching frictional engagement elements having the hydraulic control pressure introduced from the control valve by switching the oil flow passage in accordance with energizing/non-energizing an on-off solenoid valve, and an electronic control unit for controlling the energizing the linear solenoid valve and the on-off solenoid valve so as to sift the transmission by combining the engagement and disengagement of a plurality of the frictional engagement elements. In such an apparatus, there are provided a plurality of frictional engagement elements such that the engagement between a first frictional engagement element (B2) and a second frictional engagement element (C3) achieves a reverse shift, and the engagement between the second frictional engagement element (C3) and a third frictional engagement element (C1) achieves a predetermined forward shift (see Japanese Unexamined Application Publication No. 2004-116734, FIGS. 2 and 3).
In the apparatus disclosed in Japanese Unexamined Application Publication No. 2004-116734, as shown in FIG. 4, when the transmission is shifted from N range to R range (garage shift N→R), shift pattern (on-off pattern) 2 is used while when shifted from N range to D range (garage shift N→D), shift pattern (on-off pattern) 3 is used. For controlling both the hydraulic pressures applied to the frictional engagement element during the garage shift N→R (third frictional clutch C3) and the frictional engagement element during the garage shift N→D (first frictional clutch C1), a third linear solenoid valve (reference numeral 24 of FIG. 5 in Japanese Unexamined Application Publication No. 2004-116734) is used.
During the garage shift N→R, the electronic control unit (reference numeral 4 of FIG. 2 in Japanese Unexamined Application Publication No. 2004-116734) determines that the oil flow passage of the manual valve (numeral 21 of FIG. 5 thereof) be switched to R range with the position switch signal from the position sensor (numeral 45 of FIG. 2 thereof) so as to shift the automatic transmission gear from N range to R range by changing the shift pattern (on-off pattern) to pattern 2 so as to control the hydraulic pressure applied to the third frictional clutch C3 using the third linear solenoid valve (numeral 24 of FIG. 5 thereof).
During the garage shift N→D, the electronic control unit (reference numeral 4 of FIG. 2 in Japanese Unexamined Application Publication No. 2004-116734) determines that the oil flow passage of the manual valve (numeral 21 of FIG. 5 thereof) be switched to D range with the position switch signal from the position sensor (numeral 45 of FIG. 2 thereof) so as to shift the automatic transmission gear from N range to R range by changing the shift pattern (on-off pattern) to pattern 3 so as to control the hydraulic pressure applied to the first frictional clutch C1 using the third linear solenoid valve (numeral 24 of FIG. 5 thereof).
During the garage shift N→D and the garage shift N→R, although the oil flow passage of the manual valve is switched, if it cannot be determined whether the transmission is shifted N→R or shifted N→D due to the failure of the position sensor or the stoppage of the position sensor at a non-signal position, the transmission shift cannot be started because it cannot be determined to have the shift pattern (on-off pattern) for the garage shift N→D or the shift pattern (on-off pattern) for the garage shift N→R.
Hence, in the apparatus disclosed in Japanese Unexamined Application Publication No. 2004-116734, in order to start the transmission shift by detecting the manual valve whether being at D range or at R range, the electronic control unit detects the manual valve whether being at D range or at R range by providing hydraulic pressure switches in any one of or both the oil flow passage pressurized by the manual valve at D range and the oil flow passage pressurized at R range so as to detect the signal from the hydraulic pressure switches.
However, increase in the number of hydraulic pressure switches in such a manner increases the cost of the apparatus. Also, if the hydraulic pressure switch breaks, the manual valve cannot be detected whether being at D range or at R range.
Since the straight hydraulic pressure control is generally configured to control the hydraulic pressure of one engagement element by one electromagnetic valve (solenoid valve), a shift valve is not used for switching the oil flow passage. In such a case, since the electromagnetic valve for controlling the hydraulic pressure applied to the third frictional clutch C3 during the garage shift N→D is different from that for controlling the hydraulic pressure applied to the first frictional clutch C1 during the garage shift N→R, D range and R range are detected by predetermined means.
On the other hand, in the apparatus disclosed in Japanese Unexamined Application Publication No. 2004-116734, in the apparatus disclosed in the Publication, the third linear solenoid valve controls both the hydraulic pressure applied to the third frictional clutch C3 during the garage shift N→D and that applied to the first frictional clutch C1 during the garage shift N→R, the apparatus cost can be reduced by reducing the number of electromagnetic valves.
However, in the apparatus, the shift pattern (on-off pattern) 3 during the garage shift N→D is different from the shift pattern (on-off pattern) 2 during the garage shift N→R, and the position sensor determines whether the position shift is N→D or N→R with the position switch signal, so that the determination response may be delayed. Also, if the position sensor brakes, the shift pattern (on-off pattern) cannot be switched during the garage shift.